The present invention relates to apparatus for making rheological measurements and particularly to consistency transmitters.
The invention is especially suitable for use in consistency transmitters of the type which utilize a blade sensor as well as in those types of consistency transmitters which utilize a rotating sensor. The invention however is generally applicable for use in apparatus for making various types of rheological measurement in that it affords reliable and cost effective means for implementing such measurements through the use of electronic circuitry.
Blade sensor consistency transmitters determine consistency by sensing the drag imposed by the medium on a sensing blade; the medium being for example, paper stock fiber. The sensor pivots on a flexural mount, and its position is detected by a pneumatic torque transducer. A feedback system including pneumatic actuators tends to maintain the blade sensor position. Such a blade sensor consistency transmitter is available from DeZurik Corporation of Sartell, Minnesota, and is described in their Bulletin 91.001 dated Sep. 1968. The pneumatic force balance system is described in the DeZurik U.S. Pat. No. 3,285,266 and assigned to DeZurik, a unit of General Signal Corp. Another blade sensor consistency transmitter is described in U.S. Pat. No. 3,364,730 issued Jan. 23, 1968. Consistency transmitters may also have rotating sensors which are supported on flexural mounts. As consistency changes, a housing, for a motor which rotates the sensing element, itself rotates on the flexural mounts. The rotational movement of the housing can be detected by a pneumatic torque transducer and a force balance feedback arrangement may be used as described above. Reference may be had to DeZurik U.S. Pat. No. 3,285,057 and Ostroot U.S. Pat. No. 3,285,058 for further information respecting rotating sensor consistency transmitters.
While consistency transmitters utilizing pneumatic torque transducers and force balance feedback system have proven themselves entirely satisfactory in operation, it is desirable that the transducer and feedback system be implemented electronically. The problem of an electronically operating consistency transmitter is not readily solvable by reason of the stringent requirements for reliability and accuracy in the measurements under industrial operating conditions. The movements of the sensor in response to consistency changes are very minute. Electronic systems are inherently sensitive to changes in temperature and changes in pressure which can easily mask any response to consistency changes; thus rendering the measurement inaccurate and unreliable. Furthermore, the consistency measurements are affected by frictional forces which are compensated by reason of the flexural mounts for the sensors. Implementations of electronic sensors affect the mounts and contribute to errors in the consistency measurement.
Various types of electronic transducers, particularly strain gauges have been suggested for detecting flow and other fluid effects (see U.S. Pat. Nos. 2,805,574; 3,098,384; 3,115,777; 3,147,612; 3,238,773; 3,287,971; 3,338,093; 3,796,088; 3,908,458; and Re. 27,354). None of these however provide for electronically sensing consistency changes and balancing of torque in response to such changes so as to provide accurate and reliable measurements of consistency. Moreover, none of the approaches which have been suggested are concerned with the problem of avoiding any adverse effect on the mounting of a consistency sensor.